Psoriasis is a chronic inflammatory skin condition characterised by clearly defined, red and scaly plaques (thickened skin). It is classified into several subtypes.

Psoriasis affects 24% of males and females. It can start at any age including childhood, with peaks of onset at 1525 years and 5060 years. It tends to persist lifelong, fluctuating in extent and severity. It is particularly common in Caucasians, but may affect people of any race. About one third of patients have family members with psoriasis.

Psoriasis usually presents with symmetrically distributed, red, scaly plaques with well-defined edges. The scale is typically silvery white, except in skin folds where the plaques often appear shiny and they may have a moist peeling surface. The most common sites are scalp, elbows and knees, but any part of the skin can be involved. The plaques are usually very persistent without treatment.

Itch is mostly mild but may be severe in some patients, leading to scratching and lichenification (thickened leathery skin with increased skin markings). Painful skin cracks or fissures may occur.

When psoriatic plaques clear up, they may leave brown or pale marks that can be expected to fade over several months.

Certain features of psoriasis can be categorised to help determine appropriate investigations and treatment pathways. Overlap may occur.

Generalised pustulosis and localised palmoplantar pustulosis are no longer classified within the psoriasis spectrum.

Patients with psoriasis are more likely than other people to have other health conditions listed here.

Psoriasis is multifactorial. It is classified as an immune-mediated inflammatory disease (IMID).

Genetic factors are important. An individual's genetic profile influences their type of psoriasis and its response to treatment.

Theories about the causes of psoriasis need to explain why the skin is red, inflamed and thickened. It is clear that immune factors and inflammatory cytokines (messenger proteins) such is IL1 and TNF are responsible for the clinical features of psoriasis. Current theories are exploring the TH17 pathway and release of the cytokine IL17A.

Psoriasis is diagnosed by its clinical features. If necessary, diagnosis is supported by typical skin biopsy findings.

Medical assessment entails a careful history, examination, questioning about effect of psoriasis on daily life, and evaluation of comorbid factors.

Validated tools used to evaluate psoriasis include:

The severity of psoriasis is classified as mild in 60% of patients, moderate in 30% and severe in 10%.

Evaluation of comorbidities may include:

Patients with psoriasis should ensure they are well informed about their skin condition and its treatment. There are benefits from not smoking, avoiding excessive alcohol and maintaining optimal weight.

Mild psoriasis is generally treated with topical agents alone. Which treatment is selected may depend on body site, extent and severity of the psoriasis.

Moderate to severe psoriasis warrants treatment with a systemic agent and/or phototherapy. The most common treatments are:

Other medicines occasionally used for psoriasis include:

Most psoriasis centres offer phototherapy with ultraviolet (UV) radiation, often in combination with topical or systemic agents. Types of phototherapy include

Biologics or targeted therapies are reserved for conventional treatment-resistant severe psoriasis, mainly because of expense, as side effects compare favourably with other systemic agents. These include: